README: update from CC-BY-3.0 to CC-BY-4.0

CC-BY-3.0 was shiny and new back in 2009, but CC-BY-4.0 is
generally preferred now. Update our CC-BY uses to CC-BY-4.0.

Google lawyers signed off on the overall CC-BY-4.0 update
and Renee French signed off on the update of the gopher license.

See also CL 447156.

Change-Id: I3908910d6011ed733271e595f761c773351b30f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/447275
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>

README.md

@@ -4,7 +4,7 @@ Go is an open source programming language that makes it easy to build simple,
 reliable, and efficient software.
 
 ![Gopher image](https://golang.org/doc/gopher/fiveyears.jpg)
-*Gopher image by [Renee French][rf], licensed under [Creative Commons 3.0 Attributions license][cc3-by].*
+*Gopher image by [Renee French][rf], licensed under [Creative Commons 4.0 Attributions license][cc4-by].*
 
 Our canonical Git repository is located at https://go.googlesource.com/go.
 There is a mirror of the repository at https://github.com/golang/go.
@@ -39,4 +39,4 @@ proposals only. See https://go.dev/wiki/Questions for a list of
 places to ask questions about the Go language.
 
 [rf]: https://reneefrench.blogspot.com/
-[cc3-by]: https://creativecommons.org/licenses/by/3.0/
+[cc4-by]: https://creativecommons.org/licenses/by/4.0/